(1) Field of the Invention
This invention relates to the logical generation of mask patterns for outrigger type attenuating or 100% transmittance phase shifting masks.
(2) Description of the Related Art
As critical dimensions become smaller phase shifting masks are more frequently used for forming contact holes in integrated circuit wafers. FIGS. 1 and 2 show a phase shifting mask used for forming contact or via holes. FIG. 1 shows a top view of the mask 20 showing contact holes 12 formed in the phase shifting material. The phase shifting material can be attenuating phase shifting material or material having 100% transmittance. FIG. 2 shows a cross section view of the mask taken along line 2-2' of FIG. 1. FIG. 2 shows a layer of attenuating or 100% transmittance phase shifting material 14 formed on a transparent mask substrate 16. Holes 12 corresponding contact holes are formed in the attenuating phase shifting material. Masks of this type work well but can have a problem due to side lobe effect.
FIGS. 3 and 4 show an outrigger type phase shifting mask 10 used for forming contact or via holes which can avoid the problem of side lobe effect. The 10 mask shown in FIGS. 3 and 4 uses contact hole openings 12 in an opaque background 18 with outrigger bars 14 formed of phase shifting material on all four sides of the contact hole opening. FIG. 3 shows the top view of the mask 10. FIG. 4 shows a cross section of the mask 10 along line 4-4' of FIG. 3 and shows the opaque background 18 and the phase shifting outrigger bars 14 formed on a transparent mask substrate 16. The phase shifting material can be attenuating phase shifting material or can have 100% transmittance. The mask of FIGS. 3 and 4 provides good image quality for the contact hole formation and also avoids the problem of side lobe effect.
U.S. Pat. No. 5,538,833 to Ferguson et al. describes the use of a process of phase edge lithography in the fabrication of integrated circuit chips in which chrome images are biased on a phase edge of a phase shift mask and the mask is overexposed to compensate for bias.
U.S. Pat. No. 5,702,848 to Spence describes masks used in integrated circuits where pattern intersection regions are divided into categories of stacks and different phase assignment rules are employed for the different stacks.
U.S. Pat. No. 5,537,648 to Liebman et al. describes a method implemented in a computer aided design system to automatically generate phase shifted masks. The method uses a series of basic geometric operations to design areas to design areas requiring phase assignment, resolve conflicting phase assignments, and eliminate unwanted phase edges.
U.S. Pat. No. 5,807,649 to Liebman et al. describes a lithographic patterning method using a phase shift trim mask.
U.S. Pat. No. 5,308,741 to Kemp describes a lithographic method using double exposures, physical mask shifting, and light phase shifting to form masking features on a substrate masking layer.